High frequency stimulation of the subthalamic nucleus (STN) dramatically improves all of the clinical motor symptoms of Parkinson's Disease (PD). However, there are limited objective data available to determine which characteristics of movement and posture are affected by STN stimulation, and by what neural mechanisms this is accomplished. The long-term objective of this application is to obtain objective neurophysiological data relating to the mechanisms by which effective STN stimulation alters the spatial and temporal patterns of activity mediating planned movement and posture in humans. Patients in whom STN surgery is successful, as defined by a 30% reduction in the motor score of the Unified Parkinson's Disease Rating Scale, will take part in a series of experiments designed to investigate the neural control of movement and posture. The experiments in Aim I will use electromyographic (EMG) and motion analysis techniques to identify which aspects of strength, movement and standing balance are improved, worsened or unchanged by STN stimulation. The effects of STN stimulation will also be compared with the effects of medication on the control of strength and movement. The hypothesis is that neither STN stimulation nor medication normalizes the control of movement, and STN stimulation does not normalize the control of standing balance. Aim 2 will use electroencephalographic (EEG) techniques to test whether STN stimulation-induced changes in movement and gait initiation are accompanied by changes in the spatial and temporal patterns of cortical activity in response to both internally and externally generated cues to move. The hypothesis is that STN stimulation does not normalize the pathways that are normally influenced by the STN but does allow other pathways to compensate better. Aim 3 will combine EEG techniques with stimulation through the quadripolar electrodes implanted in the region of the STN to examine the pathways activated by effective STN stimulation. The findings of the proposed experiments will advance our understanding of the role of the STN in motor function, assist in the development of improved models of the role of the basal ganglia in the control of movement and posture, and thereby contribute to improved treatments for Parkinson's disease.